1. Field of the Invention
The present invention relates to a synchronous electric motor. The present invention also relates to a rotor of a synchronous electric motor.
2. Description of the Related Art
In an inner-rotor type synchronous electric motor, it is known that the outer circumferential surface of the rotor core is provided, at each of the plural poles formed in the rotor, with a contour deviating from a circular arc and bulging out in a radial outward direction, as seen in a cross-section perpendicular to a rotation axis. In the synchronous electric motor having the above rotor, a generated torque depends on the dimension of the gap between the circumferentially center region of the core outer surface in each pole of the rotor and the inner circumferential surface of a stator core (this gap being hereinafter referred to as “a center gap”). In other words, as the center gap is reduced, the torque is increased.
On the other hand, an inductance in the synchronous electric motor having the above rotor depends on the dimension of a gap between each of the circumferentially opposite lateral regions of the core outer surface in each pole of the rotor and the inner circumferential surface of the stator core (this gap being hereinafter referred to as “a lateral gap”). In other words, as the lateral gap is increased, the inductance is reduced. Then, as the inductance is reduced, the counter electromotive force generated during high-speed rotation of the rotor decreases and, as a result, the torque (or power) during the high-speed rotation increases. The reduction in inductance also reduces the energy consumption.
Therefore, in a synchronous electric motor, it has been generally required to optimize the shape of the outer circumferential surface of a rotor core at each pole of a rotor, so as to increase a torque and to decrease an inductance to improve the energy efficiency.
Japanese Unexamined Patent Publication (Kokai) No. 2002-010541 (JP-A-2002-010541) discloses a synchronous electric motor having an improved correlation between torque and inductance, in which the contour of a core outer circumferential surface at each pole of a rotor is formed into a bulging shape and in a curve represented mainly by a hyperbolic cosine function, and it is thus possible to establish a high torque at both a high-speed and a low-speed rotation. This patent Document also discloses a synchronous electric motor in which the contour of the core outer circumferential surface at each pole of a rotor is formed into a bulging shape and in a curve represented mainly by a secant function.
FIG. 8 shows an example of a rotor which adopts a bulging shape represented mainly by a hyperbolic cosine function as the contour of the core outer circumferential surface at each pole of the rotor. In the illustrated rotor, a rotor core 101 has several poles alternately formed in a circumferential direction along the outer circumferential surface 102, and is coaxially mounted onto a shaft 103. The contour of the outer circumferential surface 102 at each pole of the rotor core 101 is a bulging shape represented by a hyperbolic cosine function. The rotor core 101 is provided with plate-like permanent magnets 104, embedded therein and extending in a direction orthogonal to the radial direction of the core, one-by-one for the respective poles. Due to the bulging shape of the contour of the outer circumferential surface 102 at each pole of the rotor core 101, thinner portions 105 of the rotor core 101 are formed between the permanent magnets 104 of adjoining poles. The thinner portions 105 cooperate with a bulging portion 106 radially outside of the permanent magnet 104 to support the permanent magnet 104 against a centrifugal force.
As shown in FIG. 9, the magnetic flux coming out from one permanent magnet 104 is divided into a magnetic flux passing in the circumferential direction through the thinner portions 105 to the adjoining permanent magnet 104, and a magnetic flux passing in a radial direction through the bulging portion 106 to the stator (not shown). The magnetic flux passing through the thinner portions 105 is a leakage magnetic flux 107 that does not contribute to the generation of the torque, while the magnetic flux passing through the bulging portion 106 is an effective magnetic flux 108 that contributes to the generation of the torque. The effective magnetic flux 108 from the permanent magnet 104 usually corresponds to the remaining parts of a total magnetic flux at the instant when the thinner portion 105 is saturated with the leakage magnetic flux 107. Therefore, by decreasing the leakage magnetic flux 107 passing through the thinner portions 105, it is possible to increase the effective magnetic flux 108 flowing toward the stator. In order to decrease the leakage magnetic flux 107 passing through the thinner portions 105, it is advantageous to reduce the dimensions of the thinner portions 105.
However, during the rotation of the rotor, a centrifugal force, produced mainly in each permanent magnet 104 and the bulging portion 106 radially outside thereof, may be concentrated to the thinner portions 105, and thereby the rotor core 101 may be broken. Therefore, to ensure a sufficient rotor strength, it is required to increase the dimensions of the thinner portions 105 in the rotor core 101. Thus, for the thinner portion 105 provided in the rotor core 101, it is required to decrease the dimension thereof for increasing the torque, while it is required to increase the dimension thereof to ensure strength of the rotor.
In this connection, as shown in FIG. 10, it is also possible to increase the torque by increasing the thickness of the permanent magnet 104, without changing the dimensions of the thinner portions 105 in the rotor core 101. In this configuration, a magnetic flux increases in accordance with the increase in thickness of the permanent magnet 104 and, because the amount of the leakage flux 107 passing through the thinner portions 105 is not changed, the effective magnetic flux 108 passing through the bulging portion 106 increases, which results in an increase in torque. In this configuration, however, the increase in torque raise is not significant as compared to the increase in volume of the respective permanent magnets 104. Therefore, it is required to provide a configuration permitting the torque to be increased more efficiently.
Japanese Unexamined Patent Publication (Kokai) No. 11-243653 (JP-A-11-243653) discloses an electric motor in which a rotor includes a rotor core with a circular outer circumferential surface in cross-section and several permanent magnets embedded in the rotor core with two or more magnets per one pole. In this electric motor, each pole of the rotor is composed of the several permanent magnets having a unique configuration of magnet array, so as to increase the difference between the d-axis inductance and the q-axis inductance, and thereby it is possible to increase the reluctance torque of the electric motor. An electric motor having the similar configuration is also disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2001-095182 (JP-A-2001-095182). However, neither of the patent Documents teach nor describe a configuration such that the contour of the core outer circumferential surface at each pole of the rotor is formed into a shape deviating from a circular arc so as to permit the inductance to be decreased.